Electric resistance heating elements made of molybdenum disilicide are conventionally made in the form of an elongated loop of U-shape, of molybdenum disilicide resistance wire having legs to the ends of which molybdenum disilicide terminals are butt welded. They are commonly called hairpin elements.
The terminals are in the form of molybdenum disilicide wire and are made substantially larger in diameter than the resistance wire to reduce terminal heating when the element is in service. Electric resistance butt welding is used to weld the terminal wire ends to the resistance wire ends and this requires tapering of the terminal ends to provide ends having a diameter substantially the same as that of the resistance wire so that the butt welding is facilitated. The terminals are made long enough to accommodate their clamping by clamps which apply the electric power to the element.
Molybdenum disilicide is extremely hard, and to taper the terminal ends, requires the use of abrasive grinding techniques. The material is also brittle and this grinding is not only inherently expensive, but also involves the risk of terminal breakage.
The terminals are welded in axial alignment with the hairpin wire legs, and the terminals are vertical when clamped under service conditions so that the hairpin or loop of resistance wire depends vertically to avoid sagging problems when the element is in operation. However, for some applications it is desirable to mount the terminals horizontally with the loop of resistance wire depending, but this necessitates right angular bending of the terminal wire, because it has heretofore been impossible to weld the resistance wire right-angularly to the larger terminal wire.
The use of bent terminals requires an undesirable extent of terminal wire in the furnace using the element.
Molybdenum disilicide electric resistance wire is sold in a wide range of diameters under the trademark "KANTHAL SUPER".